Electrical signals need to be balanced to maximize power in many transmission situations. A device for balancing signals is a balun. A balun converts, for example, an unbalanced coaxial local oscillator feed signal to balanced mode, permitting the balanced mode signal to switch the diode.
In diode based frequency mixers, baluns are generally required to convert an unbalanced coaxial local oscillator feed signal to a balanced mode signal which switches the diode. Baluns may also be used on the RF input signal. The degree of electrical balance between the two balun outputs is very important in preventing input signal energy from exiting the other mixer ports. Ideally, the balun should not create large reflections or insertion losses such that the finction is degraded.
Currently in use in some 2-50 gigahertz mixers is a Marchand balun. A Marchand balun consists of two coupled sections, each one quarter wavelength long. Each section has one terminal grounded, and the load is connected between the terminals opposite the grounded ones (it can also be used, like the parallel-line balun, as a phase splitter driving two loads). The structure is surrounded by a ground plane, usually the housing in which the balun is mounted. A balun of this type has a limit of 10 to 1 maximum bandwidth. For RF signals between 20 and 50 gigahertz, the third harmonic of the local oscillator is used to mix with the RF frequencies to extend the RF frequency. Such extension, however, is at the expense of poorer conversion loss and increased noise problems.
Another balun commonly used in broadband mixers is the microstrip balun. The microstrip balun involves a tapered microstrip ground plane. Mixers with this balun provide a bandwidth of about up to about 26 to 1. This balun must be about a wavelength or longer in electrical length to function properly. As a result, the practical use is limited to microwave frequencies and higher.
Presently, in order to achieve aggregate bandwidth for broad band instruments, two or more baluns and associated mixers have been incorporated in a broadband instrument, at additional bulk and cost. As applications become increasing portable, reduction in bulk and cost are essential.
There has grown and there remains a need for baluns that are capable of performing across a broad range of frequencies, from millimeter to microwave.
What is needed is a single broadband balun operable over a range spanning millimeter wavelengths to below ten gigahertz wavelengths.